Fat-perception is thought to be related to a complex interplay between fat-associated flavor release and mouth-feel. Friction sensed between the tongue and the palate seems to play a prominent role: in previous work, we have shown that emulsions that are more sensitive toward coalescence give rise to a lowering of the orally perceived and experimentally measured friction and, probably as a consequence, to an enhanced fat-perception. In this paper, we study in detail the factors determining friction of protein-stabilized emulsions using a novel mouth-mimicking tribometer and model surfaces consisting of PDMS modified in various ways (hydrophobicity, deformability, roughness). We show that unlike in many technological applications where lubrication is essentially hydrodynamic, for physiologically relevant loads, the modified PDMS is boundary and/or mixed lubricated, which is like in-mouth lubrication. We find that an increased sensitivity of the emulsions toward coalescence results in a lower friction, confirming previous results obtained with pig's tongue. Surface-induced coalescence (or spreading of emulsion droplets) seems to be very important in this, surface hydrophobicity being the dominant trigger. Viscosity of the dispersed phase does not have such a strong influence on both the measured friction and the oral perceived friction. We do find a strong influence of the presence of bulk proteins and saliva on friction. Finally, hardly any dependence of measured friction on fat content of the emulsion was observed, indicating that only a small amount of fat is needed to alter the friction.

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An experimental design was used to study the effect of pre-drying (to 10, 15 and 20% weight loss) and par-frying conditions (160, 170 and 180 • C) on the crispness of French fries. Par-frying time was adjusted with a software program to obtain equal moisture content and internal texture for all samples. Crispness was evaluated with a sensory panel. Furthermore, samples were analysed with a texture analyser and with confocal scanning laser microscopy (CSLM). Parfrying at 180 • C resulted in a crispier product than at 160 and 170 • C. Pre-drying to 20% weight loss lead to blisters and reduced crispness in comparison with pre-drying to 10 and 15% weight loss. Instrumental texture measurements showed a good correlation with sensory crispness. Large differences in cell structure, such as blisters, could be observed with CSLM. CSLM was useful to explain results from the instrumental and sensory texture evaluation.

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